Flow cytometry is well suited to the detection of rare events. If suitable markers are available to separate the cells being analysed from the other events, as few as 1 cell in 10 7 can be measured. In order to achieve a count of the desired statistical significance, only the total number of positive events (n) is relevant. In the absence of any background, the standard deviation (SD) will be equal to √n. For a CV of 3%, 1000 positive cells need to be counted. If there are non-specific events present, this number increases. If ‘a’ events are recorded from the positive sample and ‘b’ from the negative control, the SD is √(a + b). Increasing the number of identifying markers will, generally, improve the separation of the positive cells from the bulk population and increase the precision of the measurement.